1. Field of the Invention:
This invention relates to gas sensors and more particularly to an amperometric gas sensor for detecting vapors of organophosphorous compounds in air.
2. Description of the Prior Art:
U.S. Pat. No. 4,203,726, which issued on May 20, 1980 to P. L. Patterson, describes a thermionic detector having a hardened ceramic cement bead impregnated with an alkali metal which is heated in a gaseous environment in the range of 400.degree. C. to 1000.degree. C., whereby a sample interacts with the bead to form negative ions or positive ions. The bead may contain: Na.sub.2 SO.sub.4, K.sub.2 SO.sub.4, Rb.sub.2 SO.sub.4, Cs.sub.2 SO.sub.4.
U.S. Pat. No. 4,202,666, which issued on May 13, 1980 to R. C. Hall et al., describes an alkali source 10, which is preferably a mixture of alkali salts in a silica gel matrix fused to an electrical resistance heater element.
U.S. Pat. No. 4,129,418, which issued on Dec. 12, 1978 to W. D. Davis, describes an halogen detector in which the active alkali metal compound is independently heated. Typical alkali metal compounds include alkali metal aluminate, carbonate and silicate and, in particular, sodium carbonate and rubidium carbonate.
U.S. Pat. No. 3,925,183, which issued on Dec. 9, 1975 to H. G. Oswin, describes an electrochemical cell for detecting gas which includes a heat reservoir for maintaining the electrochemical cell at substantially constant temperature.
U.S. Pat. No. 3,852,037, which issued on Dec. 3, 1974 to B. Kolb et al., describes an ionization detector including an alkali source in the form of a heated alkali-containing glass which is maintained in a heated softened state during operation of the detector.
U.S. Pat. No. 3,776,832, which issued on Dec. 4, 1973 to H. G. Oswin et al., which reissued as U.S. Pat. No. Re. 31,916 on June 18, 1985, describes a three electrode electrochemical gas sensor which can be adapted to measure oxidizable or reducible gases, such as chlorine, CO, Cl.sub.2 and hydrazine, as well as other gases. This particularly known cell has two shortcomings. First, it requires an aqueous electrolyte which has a limited service life due to evaporation of the electrolyte. Secondly, the temperature range within which the cell can operate is limited due to the possibility of freezing the electrolyte. Thirdly, a complicated structure is required to retain the liquid electrolyte in the sensor.
U.S. Pat. No. 3,677,709, which issued on July 18, 1972 to M. Riedmann et al., describes a flame ionization detector including a piece of salt supported in a collector ring. The salt may be rubidium sulfate or rubidium bromide for detecting nitrogen and phosphorous. The salt may be cesium sulfate or cesium iodine for detecting substances containing chromium chlorine, bromide and iodine.
U.S. Pat. No. 3,615,237, which issued on Oct. 26, 1971 to F. P. Speakman, describes a thermionic flame ionization detector including a flame seeding material such as of the alkali halides, for example, bromine and sulfate salts of sodium, cesium, potassium and rubidium.
U.S. Pat. No. 3,607,096, which issued on Sept. 21, 1971 C. H. Hartman, describes an alkali flame ionization detector which uses an alkali salt such as rubidium sulfate, potassium bromide, rubidium bromide, cesium bromide and potassium sulfate.
U.S. Pat. No. 3,589,869, which issued on June 29, 1971 to M. E. Scolick, describes a chemical ionization detector wherein a non-hydrocarbon constituent of the sample to be detected is reacted with an alkali metal vapor in a reaction zone by the application of heat supplied by a heater. At column 3, starting at line 56, the alkali reactive material may be one of the following alkali salts: cesium bromide, nitrate or chloride, potassium bromide or chloride, rubidium bromide, fluoride, or sulfate, or sodium bromide or chloride.
U.S. Pat. No. 3,535,088, which issued on Oct. 20, 1970 to H. Zimmermann, describes a flame ionization detector including a hollow body for holding an alkali compound such as K.sub.2 Cr.sub.2 O.sub.4 or KMnO.sub.4.
U.S. Pat. No. 3,423,181, which issued on Jan. 21, 1969 to K. P. Dimick et al., describes a flame ionization detector using a salt of an alkali or alkaline earth metal as an ion source such as cesium bromide.
U.S. Pat. No. 3,372,994, which issued on Mar. 12, 1968 to L. E. Giuffrida, describes a hydrogen flame ionization detector using a heated electrode coated with a fused alkali metal salt.
U.S. Pat. No. 2,795,716, which issued on June 11, 1957 to J. A. Roberts, describes a vapor detector embodying a positive ion source. A coating 15 of positive ion emitting material is provided on a ceramic core 12 and heated by heater coil 13. Coating 15 may be an alkali metal glass, such as alumino-silicates of the alkali metals (Li, Na, K, Rb, Cs), which provide positive ions as described in column 3 at lines 24-40.
In the publication by M. Lederer, "Chromatographic Reviews", Vol. 12 (1970), pgs. 6-39, thermionic detectors are discussed. On pages 13-16 the relationship between the sensitivity of the thermionic detector and the alkali salt used is discussed, See for example Table II on page 14.
In the publication by R. C. Hall, "CRC Critical Revs. in Analytical Chemnistry" (12/78), pgs. 323-380, an alkali flame detector is shown in FIG. 8 and discussed on pages 325-30. A flameless alkali sensitized detector is shown in FIGS. 10-11 and discussed on pages 330-344. Electrochemical detectors are discussed on pages 344-366 with a block diagram of an electrolytic conductivity detector shown in FIG. 33.
In the publication to M. E. Scolnick, "The Chemi-Ionization Detector: A Flameless Ionization Detector"--Journal of Chromatographic Sci., Vol. 8 (8/70), pgs. 462-466, FIG. 1 shows a chemi-ionization detector (CID) using cesium bromide and glass beads to provide cesium bromide vapor in the reaction zone which may be heated in the range from 800.degree. C. to 850.degree. C.
In the publication by B. Kolb, M. Auer and P. Pospisil, "Reaction Mechanism in an Ionization Detector, etc."--Journal of Chromatographic Sci., Vol. 15, (2/77) pgs. 53-63, FIG. 1 shows an alkali flame detector wherein the alkali source is a small glass bead which contains rubidium silicate.
In a publication by Kolb and Bischoff, J. Chromatog. Sci 12:625-29 (1974), a nitrogen-phosphorous detector (NPD) was briefly described.
In the publication by E. Y. Zandberg and N. I. Ionov, "Surface Ionization" (Mar.-Apr. '59), Soviet Physics Uspehki, Vol. 67(2), #2, pgs. 255-281, positive surface ionization without alkali metal atoms and alkali halide molecules on the surface of tungsten and platinum is discussed starting at pages 263-270. Positive surface ionization in electric fields is discussed on pages 270-272. Negative surface ionization is discussed on pages 274-279.
In a publication by E. Y. Zandberg, surface ionization of molecules is discussed on pages 1135 and 1136. Surface ionization in strong electric fields is discussed on pages 1136 and 1137. Surface ionization of organic compounds in weak electric fields is discussed on pages 1137-1140.
In a publication N. I. Ionov, "Surface Ionization and Its Applications," in: Progress in Surface Science, Vol. 1, Pergamon Press, New York (1972), the surface ionization of molecules is discussed on pages 301-311.
In a publication by M. Kaminsky, "Atomic & Ionic Impact Phenomena on Metal Surfaces", Academic Press, N.Y. (1965), pgs. 99-378, positive surface ionization of alkali metal atoms in weak electric fields is discussed on pages 117-124. Positive surface ionization of some alkali-salt molecules on metal surfaces in weak external fields is discussed on pages 124-127. Positive surface ionization of alkaline earth elements and compounds in weak external fields is discussed on pages 127-130.
In a publication by E. Y Zandberg and U. K. Rasulev, "Surface Ionization of Organic Compounds", Russian Chemical Revs. 51 (9), Sept. 1982, pgs. 819-832, the characteristics of surface ionization of organic compounds is discussed on pages 820-822.
In a publication by P. Kebarle, "Higher-Order Reactions--Ion Clusters & Ion Solvation", Ion-Molecule Reactions, Vol. 1, Ed. by J. L. Franklin, Plenum Press, N.Y. (1972), pgs. 315-353, gas-phase hydration of alkali and halide ions are discussed on pages 341-345.
In the publication by P. L. Patterson, "Selective Responses of a Flameless Thermionic Detector," Journal of Chromatography, 167 (1978) pgs. 381-397, a schematic diagram of a thermionic detector having a hot bead is shown in FIG. 1. The hot bead which may be an alkali metal-ceramic bead is discussed on pages 382 and 383.
In a publication by R. D. Wieting et al., "Reactions of Alkali Ions With Organic Molecules in the Gas Phase", Journal Amer. Chem. Society, 97:4, (2/19/75), pg. 924, reactions of alkali ions with organic molecules in a gas phase are discussed.
In a publication by R. H. Staley and J. L. Beauchamp, "Intrinsic Acid-Base Properties of Molecules etc.", Journal Amer. Chem. Society, 97:20, (10/1/75), pgs. 5920-5921, intrinsic acid-base properties of molecules is discussed. The binding energies of lithium ions to certain molecules is shown in FIG. 1.
U.S. Pat. No. 4,184,937, which issued on Jan. 22, 1980, to H. Tataria et al. entitled "Electrochemical Cell for the Detection of Chlorine", describes a three electrode electrochemical cell with a non-aqueous electrolyte consisting preferably of lithium perchlorate dissolved in an organic solvent selected from the group consisting of gamma-butyrolactone and propylene carbonate. The non-aqueous electrolyte has a considerably lower freezing point and vapor pressure than an aqueous electrolyte. The electrodes for use in the two or three electrode electrochemical cells are comprised of either gold or platinum black.
In a publication by J. A. Plambeck, published in Electroanalytical Chemistry, Wiley-Interscience, pages 50-51, New York, N.Y. (1982), a potentiostat is described for maintaining a sensing electrode of an electrochemical cell at a fixed potential with respect to its reference electrode.
It is therefore desirable to use an electrolyte in an electrochemical gas sensor which is solid and operable over a wide range of environmental conditions.
It is further desirable to provide a solid electrolyte in an electrochemical gas sensor which is stable, will not decompose, has a high boiling temperature and a low vapor pressure in the liquid and solid state.
It is further desirable to provide an electrolyte in an electrochemical gas sensor which provides structural support to electrodes.
It is further desirable to provide a solid electrolyte in an electrochemical gas sensor which includes an alkali salt embedded in a refractory cement or ceramic matrix to provide a structurally self-supporting structure.
It is further desirable to provide a solid electrolyte including an alkali salt in an electrochemical gas sensor to improve the specificity of the gas sensor.
It is further desirable to reduce the complexity of prior art electro-chemical gas sensors by replacing the liquid electrolyte, and its associated seals and reservoir, with a solid electrolyte.